It has been known for forty years to achieve transverse tentering by passage of the film between grooved rollers wherein each groove extends substantially helically or circularly around each roller and consists of a base, a peak at each side, and side walls that extend between the base and the peaks. The intermeshing of the peaks, as the rollers come together in the nip, causes the material to be stretched transversely into pleats that extend substantially in the machine direction. Depending upon whether the grooves are circular or helical, the pleats will extend diagonally or substantially exactly along the machine direction.
In all these methods the film is relatively freely suspended between the peaks even at the point of greatest intermeshing. If the peak is narrow relative to the film thickness the effect of the peak is to cause stretching mainly on the peak, but if the peak is relatively broad then stretching occurs irregularly between the peaks. Accordingly the process is normally repeated several times but still yields a profiled product and the profile is then non-uniform.
In some processes the side walls of the grooves are outwardly inclined. For instance in JP-B-4829386 such an apparatus is shown in which the peak height is 10 mm, and the separation of adjacent peaks appears also to be about 10 mm. In each groove the film is squeezed between the base and the peak (having a radius of curvature of 4 mm) of the corresponding groove but the film apparently is not under compression at other positions within the groove. Thus it is not squeezed between the side walls. The main purpose of this step seems to be uniaxial orientation, since biaxial orientation is achieved in the overall process (and in the process of JP-B-4619831) by passing the sheet material subsequently through intermeshing grooved rollers in which the grooves extend axially. Transverse stretching by intermeshing grooved rollers in which the side walls are outwardly inclined is also described in Schwartz U.S. Pat. No. 4,116,892.
All the stretching is caused by the peaks and the outwardly extending side walls play substantially no part in the process. A design that has generally been found more satisfactory has vertical side walls, thereby leaving a greater amount of space for free movement of the film between intermeshing peaks. Such apparatus is described in GB 1,526,722, 1,526,723 and, especially, 1,526,724.
Unfortunately in such apparatus the peak separation cannot be too small, e.g., less than about 2 mm, as otherwise there is substantial risk of mechanical damage to the peaks. Also only relatively thin polymeric sheet materials (e.g., below 200 or 300 g/m.sup.2) can be handled when the peaks are close, e.g., about 2 mm or a little more.
It would be desirable to reduce the risk of damage to the peaks and to be able to obtain a more uniform profile and/or to be able to stretch heavier material. It would also be desirable to be able to achieve more uniform effect on the sheet material in a single pass than has been obtainable previously. These objectives would be desirable not only for transverse stretching of single sheet material but also for fibrilation of appropriate sheet materials and for bonding laminates.